1. Field of the Disclosure
The present invention relates to liquid crystal display devices, and more particularly, to a liquid crystal display device which can sense a change of static capacitance of liquid crystals caused by a touch to detect the touch and a position of the touch.
2. Discussion of the Related Art
Recently, as the times turns to a full scale information oriented age, a field of display devices which express an electric information signal visually has been developed rapidly, and, keeping pace with this, a variety of flat display devices which are thin, and light and have an excellent performance of a low power consumption have been developed, replacing the present cathode ray tubes, quickly.
As specific examples of such flat display devices, there are liquid crystal display devices LCD, plasma display panel devices PDP, field emission display devices FED, electro luminescence display device ELD, and so on, which have a flat display panel in common essentially for displaying a picture. The flat display panel has one pair of transparent insulating substrates bonded to face each other with a unique light emission material or a polarizing material layer disposed therebetween.
Of the flat display devices, the liquid crystal display device displays a picture by controlling light transmissivity of liquid crystals by using an electric field. To do this, the liquid crystal display device is provided with a display panel having liquid cells, a back light unit for directing a light to the display panel, and a driving circuit for driving the liquid cells.
The display panel has a plurality of gate lines and a plurality of data lines crossing each other to define a plurality of pixel regions. At each of the pixel regions, there are a thin film transistor array substrate and a color filter array substrate arranged to face each other, spacers for maintaining a fixed cell gap between two substrates, and liquid crystals filled in the cell gap.
The thin film transistor array substrate is provided with the gate lines and the data lines, a thin film transistor form at every crossed portion of the gate lines and the data lines as a switching device, pixel electrodes and so on each formed for each liquid cell connected to the thin film transistor, and an alignment film coated thereon throughout the thin film transistor array substrate. The gate lines and the data lines receive signals from the driving circuits through pads for gate lines and the data lines, respectively.
The thin film transistor supplies a pixel voltage signal supplied to the data line to the pixel electrode in response to a scan signal supplied to the gate line.
The color filter array substrate is provided with a color filter formed for each liquid crystal cell, a black matrix for division and reflection of an external light, a common electrode for supplying a reference voltage to the liquid crystal cells commonly, and the alignment film coated thereon throughout the color filter array substrate.
Fabrication of the display panel is finished as the thin film transistor array substrate and the color filter array substrate fabricated separately thus are aligned and bonded together, and liquid crystals are injected into and sealed between the two substrates.
Recently, requirements for addition of a touch panel to the liquid crystal display device fabricated thus are increasing, which enables to recognize a touch portion with a human hand or an input means, and provide additional information in response to the recognition. Currently, since the touch panel is applied to liquid crystal display device in a form in which the touch panel is attached to an outside surface of the liquid crystal display device, there have been efforts for mounting the touch panel in the panel of the liquid crystal display device.
The following is an example of the touch panel mounted in the liquid crystal display device for preventing a volume of the liquid crystal display device from increasing due to the attachment of the touch panel.
FIG. 1 illustrates a circuit diagram of a related art capacitance system schematically, and FIG. 2 illustrates a circuit diagram of the capacitance sensor in FIG. 1 and a driving system thereof.
Referring to FIGS. 1 and 2, a related art liquid crystal display device is provided with first and second substrates (not shown) facing each other, a liquid crystal layer (not shown) filled between the first and second substrates (not shown), a gate line Gate 11 and a data line Data 12 formed on the first substrate to cross each other to define a pixel region, and a thin film transistor TFT formed at a crossed portion of the gate line 11 and the data line 12. Formed on the second substrate, there is a common electrode (not shown) Vcom (an applied voltage) throughout an entire surface thereof, and, formed on the first substrate, there is a pixel electrode 13 at a pixel region of the first substrate.
In addition to this, the related art liquid crystal display device is provided with a first line 21 formed parallel to the gate line 11 on an outer side of the pixel region for sensing the capacitance, a second line 22 formed parallel to the data line 12, and a first reference voltage line Vref1 and a second reference voltage line Vref2 formed parallel to the first line 21 and the second line 22, respectively.
Formed between the first reference voltage line Vref1 and the first line 21, there is a first supplementary capacitor Cref1, and formed between the first reference voltage line Vref1 and the common electrode Vcom, there is a first capacitance capacitor Clc1. In this case, the first supplementary capacitor Cref1 and the first capacitance capacitor Clc1 are connected in series. The first supplementary capacitor Cref1 and the first capacitance capacitor Clc1 connected in series thus are formed for each pixel.
Alikely, formed between the second reference voltage line Vref2 and the second line 22, there is a second supplementary capacitor Cref2, and formed between the common electrode Vcom and the second line 22, there is a second capacitance capacitor Clc2. In this case too, the second supplementary capacitor Cref2 and the second capacitance capacitor Clc2 are connected in series.
By providing an amplifier 31 at a terminal as shown in FIG. 2, what is obtained of a value of a signal applied to the first line 21 is an amplified value of a voltage at a node Vn1 between the capacitance capacitor Clc 32 and the supplementary capacitor 33, for determining if there is a touch and to sense a touch position according to the amplified value. That is, a value of the capacitance capacitor Clc 32 changes if there is the touch, and, different from an initial state, the value of the capacitance capacitor Clc 32 when being touched is measured as a voltage Vout from the node Vn1 through the amplifier 31, thereby sensing a touch state and a touch position.
At the other side which is an opposite side of an output side of the capacitance capacitor and the node Vn1 of the supplementary capacitor, first and second switches sw1 and sw2 are provided for applying signal to the first and second switches sw1 and sw2, selectively.
Two common voltage values Vcomh and Vcoml are respectively applied to the first and second reference voltage lines Vref1 and Vref2 crossing each other, which are connected to one side of the first and second supplementary capacitors Cref1 and Cref2 33. When a common voltage is VcomH, a voltage Va is supplied through a first switch sw1, stored at the Clc1 32, and provided to the amplifier 31 when the common voltage is Vcoml. At the end, the voltage provided thus includes information on a Clc32 value changed thus at the time of the touch. Change of an output voltage caused by the change of the capacitance can be expressed as follows.
            ∂              V        nl                    ∂              C        LC              =                    C        ref                              (                                    C              ref                        +                          C              LC                                )                2              ·          (                        V          comH                -                  V          comL                    )      
In such a configuration, wiring crossing an X-axis and Y-axis is required, and according to this, an increased parasitic capacitance is foreseen.
However, the related art liquid crystal display device which recognizes the touch by the capacitance capacitor system has the following problems.
First, since whether the touch is made or not is sensed by sensing a voltage change of one pixel point selectively, the recognition is impossible if a plurality of pixel points are touched.
Second, the wiring is crossed for sensing touch positions at the X-axis and the Y-axis, increasing a size of the panel, and, as consequence of the increased panel size, line resistance of the wiring, and parasitic capacitance between lines are increased, to increase coupling capacitance that drops an S/N (Signal to Noise) ratio, making reliability of the signal poor which makes recognition of the touch difficult.